The pair will open up for the first time on the relationship they had with their mum, the “People’s Princess” in a new documentary next week.

The program, Diana, Our Mother: Her Life and Legacy, will mark the 20th anniversary since Diana’s death in a car crash in Paris on August 31, 1997.

It will be aired on ITV1 on Monday, July 24 at 9pm and see the two brothers speak candidly about their mum as they look through the private family photo album.

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Princess Diana was one of the best loved members of the Royal family. Her tireless dedication to charity and her family secured her position in the hearts of the British public. The nation was shocked when she was tragically taken before her time in a car crash in Paris

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Glancing at Prince Charles on a visit to Prince Edward Island in Canada

The 90-minute film also includes photos of Diana never seen before, taken from an album that Diana had assembled, as well as special interviews from the Princess’ family and friends.

At one point in the documentary, while studying a photo of Diana holding him while pregnant with Prince Harry, Prince William says: "Believe it or not, you and I are both in this photograph, you’re in the tummy!”

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Princess Diana with Lady Sarah Armstrong-Jones

He adds: “She was very informal and really enjoyed the laughter and the fun.

“But she understood that there was a real life outside of Palace walls.”

Harry says in the film: “This is the first time that the two of us have ever spoken about her as a mother.

“She was one of the naughtiest parents.

“She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world. She smothered us with love, that’s for sure.”

ITV

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Judi James

The documentary comes as a body behaviour expert, who has seen hours of footage of the Royal Family, reveals to Daily Star Online that Harry is still a “grieving little boy”.

After looking through previous footage of Harry talking about his mum, expert Judi James says the young Prince’s emotions have not changed much since he was a child.

She told us: “Harry’s responses when he speaks of his mother do sadly look a lot more intense and childlike as a result.

“Harry had no family roles apart from grieving small boy and he still hasn’t, meaning his responses might have changed less with time.”

The documentary, which comes after Camilla’s 70th birthday during which we revealed she once cosied up to Diana, also celebrates the achievements of Diana’s work and her sons’ determination to continue the campaigns that were closest to her heart.

The film’s Executive Producer at Oxford Film and Television, Nick Kent, said: “This film will show Princess Diana in a way she has never been seen before, through the eyes of the two people who knew her best.”